Changes in Range of Motion Envelope of the Back Due to Repetitive Lifting

CHANGES IN RANGE OF MOTION ENVELOPE OF THE BACK DUE TO REPETITIVE LIFTING

Muhammad I Gul (1), Timothy Craig (2), Neena Sharma (3), Sara E Wilson (2)

(1) Chemical & Petroleum Engineering University of Kansas Lawrence, Kansas USA

(2)   Mechanical Engineering University of Kansas Lawrence, Kansas USA

(3) Physical Therapy and Rehabilitation Sciences University of Kansas Medical Center Lawrence, KS USA

INTRODUCTION

         Low back pain (LBP, including pain in the lumbosacral region of the spine and sciatic pain radiating to the legs) is a common and costly public and occupational health problem that has been associated with the performance of repetitive lifting tasks in the workplace [1-2]. Occupations that involve performing cyclic loading tasks have been identified to cause a 10-fold increase in low back injury [1]. Studies in animals and humans have shown that repetitive cyclic loading of ligaments of the spine can alter the range of motion (ROM) of the lumbar spine [3-5]. These studies have further shown that such changes can affect the dynamic control of spine motion, potentially increasing risk of low back injury [3].  The objective of this study was to examine effects of repetitive lifting might have on the range of motion envelope that describes the limits of lumbar motion.

         A previous study done in the Human Motion and Biomechanics Laboratory at The University of Kansas identified the importance of looking at not only minimum and maximum torso flexion postures, but also the local lumbar angle limits as a function of moderate torso flexion in order to develop better understanding of the ROM envelope [6].  For each torso flexion angle, subjects are asked to flex and extend their lumbar spine while maintaining the same torso flexion to achieve a range of lumbar angles.  This range is an envelope through which subjects can move during a lifting task.  Since lifting tasks operate within this ROM envelope, it is important to understand this envelope and how it might change with exposure to injury risk factors such as repetitive lifting.

         In this study, the specific aim is to examine change in the lumbar ROM envelope with a moderate cyclic lifting task.  The hypothesis was that the upper (more kyphotic) limits of the ROM envelope would increase and the lower (more lordotic) limits of the ROM envelope would decrease after the cyclic lifting task.

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Figure 1. In this study, the range of motion envelope describes the maximum and minimum achievable lumbar angle for each torso flexion angle.  Previous studies of lumbar range have focused on examining lumbar angle and torso flexion angle at maximum torso flexion.

METHODS

         Nine healthy adult subjects (5 male, 4 female, 24.8±4.4 years, 1.72±.01m) were recruited for this study. Lumbar angles were recorded by attaching a set of 6-D electromagnetic motion sensors (Trakstar, Ascension Technology, Burlington, VT) to the skin at the T10 spinous process, S1 sacrum, and manubrium using a double sided adhesive tape. Each sensor returns orientation of the sensors in Euler angles (angle sequence of flexion, lateral bending, rotation) and position.  This data was acquired at 100 Hz and used in a biofeedback display to allow subjects to observe their torso flexion and lumbar angle.  To determine torso flexion, a line connecting the T10 and S1 sensor positions was examined and torso flexion was determined as the angle of that line from vertical in the sagittal plane.  This angle represents the tilt of the trunk.  Lumbar angle was determined by comparing the orientation of the T10 and S1 sensors.  Specifically, the lumbar angle was the difference in the flexion Euler angle of the T10 and S2 sensors.

         For the ROM envelope, the minimum and maximum lumbar angle (dependent variable) at four different flexion angles (independent variable) was measured.  A biofeedback display created in Labview (National Instruments, Austin, TX) was used to display lumbar angle and torso flexion.  Subjects were asked to flex their trunk to a target torso flexion and then hold that torso flexion angle while arching and flexing their low back to obtain the maximum and minimum attainable lumbar angles. Subjects were allowed to practice this movement until they were comfortable with the task. They were then asked to repeat the maximum and minimum lumbar angles three times for each of four torso flexion angles 0¼, 30¼, 60¼ and 80¼.

         After the ROM envelope was assessed, the subjects were asked to repetitively lift a crate with handholds using straight legs for four minutes. A weight of approximately 3% of the subject's maximum lifting force was placed in the crate.  Subjects were instructed to lift at a rate of fifteen lifts per minute based on audio cues.  Subjects were asked to raise the crate from floor level to waist level, pause and then lower the crate to the floor for one lifting cycle.  After the 4 minute cyclic lifting task, the subjects were asked to repeat the ROM envelope assessment.

         A repeated measures ANOVA was used to assess changes in the lumbar angles limits in the ROM envelope before and after the cyclic lifting task.  Other independent variables in the ANOVA included trunk flexion angle and limit (maximum or minimum).

RESULTS

         In this study, the lumbar range of motion at 0°, 30°, 60° and 80° of torso flexion was assessed before and after a cyclic lifting task. A repeated measures ANOVA of the lumbar angles in the range of motion envelope found the lumbar angles were significantly different between the two limits (maximum versus minimum) as expected and between the trunk flexion angles. No significant change was observed with time (pre versus post cyclic lifting tasks) (p<0.05). The mean range of motion limits for the lumbar angle is shown in Figure 1.

DISCUSSION

         Recent studies have shown that cyclic loading of lumbar spine increases the laxity of the joints thereby increasing the range of motion of the back [3-5]. Solomonow et al. argued that such changes in lumbar spine ROM could lead to increased instability in the musculature of lower back and hence increase risk of developing LBP and other low back complications [4].  These authors observed that increased range of motion in the spine due to cyclic load in a cat model led to reduce stretch reflex activation supporting this theory.  However, these past studies have only examined the extreme limits of torso flexion rather than the range of motion of the lumbar spine and low back at moderate torso flexion angles where lifting tasks might normally occur.

         The results of this study reveal that performing the cyclic task in this study for four minutes did not significantly change the ROM envelope of the lower back. This finding is different from previous studies of lumbar range of motion with cyclic lifting.  The first potential reason for this difference is that the ROM envelope captures primarily local lumbar flexibility rather than a global trunk flexibility (that might also incorporate characteristics of other joints such as the hip).  The second potential reason is that the cyclic lifting task in this study was a fairly moderate task with a light load and moderate torso flexion excursion. To fully assess the effects of cyclic lifting on the ROM envelope, the next step of this study will be to examine how the ROM envelope changes when subjects are exposed to a longer period of cyclic lifting task, a heavier load task, and/or a higher torso flexion lifting task. 

ACKNOWLEDGEMENTS

         Research reported in this publication was supported by the National Institute of Arthritis and Musculoskeletal and Skin Disease of the National Institutes of Health under Award Number 1R03AR061597-01. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

REFERENCES

[1] McGill S,. J Biomech 30: 465–75, 1997.

[2] B. P. Bernard, "Musculoskeletal disorders and workplace factors," U.S. Department of Heath and Human Services, Washington, D.C. 97-141, 1997.

[3] M. Solomonow, et al., Clin Biomech, 18: 273-9, 2003.

[4] Solomonow, M. et al., Spine, 24(23): 2426-2434, 1999.

[5] S. M. McGill, et al., Ergonomics, 39: 1107-18, 1996.

[6] A. Maduri, et al., J Electromyogr Kinesiol, 18: 807-14, 2008.

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Figure 2. Subjects were asked to flex and extend their back before and after the cyclic lifting task. The figure shows an average minimum and maximum Lumbar angle plotted for 0°, 30°, 60° and 80° flexion angles. The shaded region shows standard deviation.